A non-return valve (hereinafter referred to as the “valve”) is installed on a tip of a melt processing screw (hereinafter referred to as the “screw”). The screw is mounted in a molding machine barrel (hereinafter referred to as the “barrel”). The screw rotates and forces melt forwardly which forces the valve to open and receive forwardly-advancing melt. Once enough melt is accumulated, the screw is then translated forwardly to force the valve to close. Then the screw is accelerated forwardly which causes the accumulated melt to surge out from the barrel and into a mold cavity. While the screw moves forwardly, the valve should, ideally, remain in a closed position which prevents a back flow of melt back towards the screw. Hence, the term “non-return” means that the valve prevents the melt from flowing back towards the screw as the melt is moved or injected into the mold cavity. Known valves attempt to prevent backflow but do so with less than ideal results.
Known valves are described in U.S. Pat. No. 6,007,322 (published in 1999), U.S. Pat. Nos. 5,756,037, 5,112,213, 4,988,281, 4,643,665, 4,105,147, 3,726,309, 3,590,439 and 3,344,477 (issued in 1967). Known valves, for at least 30 years, have suffered and continue to suffer from a high shot-to-shot variability (hereinafter referred to as the “shot variability”). In other words, each shot injected into a mold cavity is different from each other in volume and/or weight. It is desired to have a low shot variability, in that each injected shot is substantially repeatable in size by volume and/or by weight. Another indicator that is used to indicate valve performance is shot “repeatability”. Known valves suffer from low shot repeatability. If shot size varies, the molded articles are not filled with an optimum amount of weight and/or volume of melt. Also, it is believed that if shot sizes vary, then the injection pressure “profiles” (that is, the pressure profile is a change in the injection pressure during injection of the melt over an injection cycle time) will vary which then will have a negative impact on article quality.
Several known theories for resolving the problem of shot variability are currently promoted. One theory suggests that to resolve the problem of low shot repeatability, molding machines should include the use of a closed-loop injection unit control, either with servo-electric valves on a hydraulic press or AC servomotors on an all-electric press. Yet another theory suggests that to resolve the problem, molding machines should include screws designed to meet the requirements of the melt and of the motor output that drives the screw. These theories attempt to resolve the high shot variability problem; however, over a span of over 30 years, the problem appears to persist and continue without a satisfactory outcome on the horizon.